Field of the Invention
The present invention relates to a narrow directional microphone using an acoustic tube, and especially relates to a narrow directional microphone that can decrease a wind noise and a vibration noise.
Description of the Related Art
A narrow directional microphone using a long and narrow acoustic tube and having a microphone unit arranged in an inner bottom portion of the acoustic tube has been known, and this narrow directional microphone has high sensitivity characteristics for a sound wave entering from a front end portion in an axial direction of the acoustic tube.
That is, in the acoustic tube, the front end portion thereof is open, for example, slit-like sound wave introduction ports are formed in a side surface, and an acoustic resistance material is stuck to the sound wave introduction port in the side surface. With this configuration, among sound waves from directions other than the axial direction of the acoustic tube, a sound wave entering from the front end portion of the acoustic tube and a sound wave entering through the sound wave introduction port in the side surface of the acoustic tube have a time difference, and interfere with and cancel each other.
As a result, a narrow directional microphone having high sensitivity only for the sound wave from the front end portion in the axial direction of the acoustic tube can be obtained, and this is also called a shotgun microphone.
In the narrow directional microphone using the acoustic tube, since the microphone unit is positioned in the inner bottom portion of the acoustic tube, a distance between a front acoustic terminal and a rear acoustic terminal of the microphone unit becomes long, and thus a wind noise increases.
Further, a vibration noise increases with the mass of the air in the acoustic tube in the narrow directional microphone using the acoustic tube, and thus there is a technical problem that the vibration noise becomes larger as the acoustic tube is longer.
Since these wind noise and vibration noise are mainly generated in a low frequency band, measures are considered to include a low frequency cut filter that removes a low frequency range component from an audio signal from the microphone unit, and to insert the low frequency cut filter into a transmission path of the signal from the microphone unit by a manual switch operation, when the wind noise and the vibration noise are anticipated to be large.
Meanwhile, the applicant of the present application has proposed a microphone including means for decreasing a vibration noise, and which is disclosed in Japanese Patent No. 4000217 B2 (a patent document 1).
According to the microphone disclosed in the document 1, a vibration detection unit using a piezoelectric element is included in a microphone case, and the microphone operates to attenuate and output an audio signal from the microphone unit based on a vibration detection signal from the vibration detection unit.
Accordingly, a vibration noise can be reduced which, for example, is generated on the basis of a relative speed between a vibration system including a diaphragm and a fixed portion such as a magnetic circuit.